Mapping the Serotonergic System : Topographical Organization of Serotonergic Projections from the Dorsal Raphe Nucleus

Abstract

The serotonin system is considered one of the most complex neuromodulatory systems in the central nervous system and is one of the most frequently targeted systems for pharmacological studies addressing a wide range of psychiatric dysfunctions. Serotonergic neurons are topographically organized with anatomically distinct groups receiving synaptic input from specific brain structures, suggesting organization in both function and projection output to a wide range of forebrain and brainstem circuits. There still remains much confusion about the role of serotonin, and many of the functions effected by serotonin modulation, including affective control, behavioral inhibition and sensorimotor gating, may be linked to the structural organization mapped within the system. The aim of this project is to develop and implement labeling methods to visualize and control specific projection pathways of serotonin neurons originating in the dorsal raphe nucleus. Retrograde labeling tools were explored and optimized. We were able to optimize the experimental protocol for retrograde labeling using latex microspheres, and the results confirmed that there are projections from the dorsal raphe nucleus to both the piriform cortex and the olfactory bulb with a majority of these neurons being serotonergic. Piriform cortex projecting cells were more numerous and clustered in the posterior portion of the dorsal raphe region. Two types of retrograde viruses were also tested, though with less success and efficiency. They do, however, offer future possibilities for research exploring the structural and functional anatomy of the serotonin system.